14 Peripheral Nerve Blocks at the Ankle
The foot is supplied by five nerves. Four of them are derived from the sciatic nerve (tibial nerve, superficial fibular nerve, deep fibular nerve, and sural nerve). The sural nerve is a joint terminal branch with sensory nerve fibers from the fibular nerve and the tibial nerve. The fifth nerve, the saphenous nerve, is the sensory terminal branch of the femoral nerve from the lumbar plexus (Chapter 8).
Three of the five nerves run in the subcutaneous fat directly above the deep fascia of the leg: the saphenous, sural, and superficial fibular nerves. These are anesthetized by subcutaneous infiltration. The remaining two nerves—the tibial nerve and the deep fibular nerve—run below the deep fascia of the leg in the ankle region and are anesthetized by selective subfascial injections (Fig. 14.1; Platzer 2009).
Each of the five nerves in the ankle can be blocked separately. The success rate is very high.
14.1.1 Tibial Nerve
The tibial nerve (L4–L5 to S1–S3) is the larger of the two branches of the sciatic nerve and in the distal segment of the lower leg it becomes superficial medial to the Achilles tendon. It lies behind and lateral to the posterior tibial artery and between the tendons of flexor digitorum longus and tibialis posterior and the flexor hallucis longus muscles, covered by the flexor retinaculum (Fig. 14.1, Fig. 14.2, Fig. 14.3, Fig. 14.4). The tibial nerve always runs close to the posterior tibial vessels. The tibial nerve gives off medial calcaneal branches on the inside of the heel and then divides behind the medial malleolus into the medial plantar nerve and the lateral plantar nerve. These two nerves run downward, toward the sole of the foot, covered by abductor hallucis, and provide the sensory innervation of the sole. The tibial nerve is the motor supply of the flexor muscles (plantar flexion) and the sensory supply of the anterior and medial regions of the sole.
14.1.2 Saphenous Nerve
The saphenous nerve is the sensory terminal branch of the femoral nerve. It passes through the deep fascia of the leg in the region of the pes anserinus at the medial knee joint line and runs distally very close to the long saphenous vein on the medial side of the tibia subcutaneously, reaches the ankle region anterior to the medial malleolus, and continues as far as the great toe, giving off branches on the medial border of the foot (Fig. 14.1, Fig. 14.2, Fig. 14.3, Fig. 14.5). The sensory innervation of the medial region of the heel, the medial malleolus, and the medial border of the foot, sometimes as far as the great toe, is provided by the saphenous nerve.
14.1.3 Sural Nerve
The sural nerve is a cutaneous nerve that is a branch of the tibial nerve, which unites with the fibular communicating branch of the lateral sural cutaneous nerve (from the common fibular nerve). The nerves usually unite in the middle third of the lower leg. In the further course it passes through the fascia. The sural nerve (which is also called the external saphenous nerve) then runs in the subcutaneous layer and passes downward together with the short saphenous vein on the deep fascia of the leg behind the lateral malleolus to the outer edge of the foot (Fig. 14.6 and Fig. 14.7). It is the sensory supply of the lateral heel region and lateral malleolus and, as the lateral dorsal cutaneous nerve, it innervates the lateral border of the foot as far as the small toe.
14.1.4 Superficial Fibular Nerve
The superficial fibular nerve comes from the common fibular nerve and branches off in the fibular compartment or even sooner. It passes cutaneously, either as a trunk or already divided into its two terminal branches, between the middle and distal third of the lower leg lateral to the anterior edge of the tibia. Lying directly on the deep fascia of the leg, it passes distally and branches above ankle level in a broad fan shape over the entire dorsum of the foot, for which it is the sensory supply with its terminal branches, namely the medial and intermediate cutaneous dorsal nerves (Fig. 14.6, Fig. 14.7, Fig. 14.8, Fig. 14.9). The superficial fibular nerve is also called the musculocutaneous nerve of the leg (Bridenbaugh and Wedel 1998).
14.1.5 Deep Fibular Nerve
The deep fibular nerve passes downward on the anterior surface of the interosseous membrane of the leg deep to the extensor digitorum longus muscle. It is always lateral to the anterior tibial artery between the tibialis anterior and the extensor digitorum longus and extensor hallucis longus muscles that are located laterally.
It then continues along the dorsum of the foot, covered by the superior and inferior extensor retinaculum. There it innervates the short toe extensors and the skin on the lateral side of the great toe and the medial side of the second toe—that is, the first interdigital space. During its course in the lower leg, the tibial artery is, at first, medial to the nerve. However, further distally the nerve crosses above the artery, which then lies lateral to it. At the level of the extensor retinaculum the nerve and artery are crossed by the tendon of extensor hallucis longus coming from the medial side. At the front of the ankle joint, the anterior tibial artery is therefore lateral to the deep fibular nerve, while the tendon of extensor hallucis longus is medial to the nerve (Fig. 14.6, Fig. 14.8, Fig. 14.10).